The structurizer is implemented along the lines of what is usually called
the "structured program theorem": the control flow is completely
virtualized by mean of an additional TEMP register which stores the
block index which is currently running. The whole program is then
converted to a huge switch construction enclosed in a loop, executing
at each iteration the appropriate block and updating the register
depending on block jump instruction.
The algorithm's generality is also its major weakness: it accepts any
input program, even if its CFG is not reducible, but the output
program lacks any useful convergence information. It satisfies the
letter of the SPIR-V requirements, but it is expected that it will
be very inefficient to run on a GPU (unless a downstream compiler is
able to devirtualize the control flow and do a proper convergence
analysis pass). The algorithm is however very simple, and good enough
to at least pass tests, enabling further development. A better
alternative is expected to be upstreamed incrementally.
Side note: the structured program theorem is often called the
Böhm-Jacopini theorem; Böhm and Jacopini did indeed prove a variation
of it, but their algorithm is different from what is commontly attributed
to them and implemented here, so I opted for not using their name.
For simplicity PHI nodes are not currently handled.
The goal for this pass is to make the CFG structurizer simpler, because
it doesn't have to care about the more rigid rules SSA registers have
to satisfy than TEMP registers.
It is likely that the generated code will be harder for downstream
compilers to optimize and execute efficiently, so once a complete
structurizer is in place this pass should be removed, or at least
greatly reduced in scope.
PHI nodes must be fixed up after this pass, because the block references
might have become broken. For simplicitly this is not handled yet.
The goal for this pass is to make the CFG structurizer simpler, because
only conditional and unconditional branches must be supported.
Eventually this limitation might be lifted if there is advantage in
doing so.
